1. Field of the Invention
The invention is in the field of magnetic resonance measurements particularly as related to the determination of magnetic susceptibilities.
2. Description of the Prior Art
Magnetic resonance has long been a useful technique for measuring a variety of parameters associated with bulk materials such as, for example, spin/lattice relaxation times, spin/spin relaxation times, self-diffusion coefficients, sample densities, nuclear magnetic moments, nuclear quadrupole moments, spins, and magnetic susceptibilities. Fundamental to the nuclear magnetic resonance phenomenon are the energy level transitions of nuclei resulting from induced emission or absorption by the sample nuclei placed in a rf field. Magnetic resonance has been theoretically described both in classical and quantum mechanical terms and many experimental techniques have been devised which by and large confirm theoretical predictions. Detailed considerations of theoretical and experimental approaches to magnetic resonance may be found in the literature, as for example, The Principles of Nuclear Magnetism by A. Abragam and Nuclear Magnetic Resonance by E. R. Andrew.
Nuclear magnetic resonance techniques have long been utilized for the determination of various parameters such as nuclear magnetic susceptibilities. Typical of these measurements are rf bridge techniques wherein sample nuclei within a multiturn coil in a LC circuit are placed in parallel with a dummy tuned circuit. The sample nuclei resonance unbalances the bridge to produce an output signal whose amplitude is measured and recorded. Typically, the absorption or dispersion curve may be determined. Other techniques utilize a marginal oscillator approach wherein a sample coil and condenser form a parallel tuned circuit within a rf oscillator. The absorption spectrum may then be determined by sweeping the magnetic field through resonance and measuring the voltage amplitude of the oscillator. Optionally, the magnetic field may be fixed and the oscillator frequency varied to achieve a resonance. In both the bridge and maginal oscillator techniques the output signal is a voltage level, and the variation of the voltage level is measured to detect the resonance. Although such experiments are useful for the determination of relative nuclear susceptibilities, they inherently lack a means for accurately calibrating the system so that accurate or "absolute" nuclear susceptibilities may be determined. It is not possible to calibrate these susceptibility measurements accurately since the exact amount of energy absorbed in the sample depends on various circuit parameters which are not well-defined including, for example, the non-linearity of the oscillator and the gain of the amplifier and/or detector utilized. Amplifier gain, or more generally the gain of the measuring electronics as a whole, often shifts in time preventing any accurate determination of magnetic susceptibilities. Additionally, the multi-turn coils utilized to surround the sample nuclei impose problems in the determination of filling factors and demagnetization factors which further add to the errors in any "absolute" measurement.
Experiments have been conducted measuring the frequency jump in the NMR oscillator as a means for determining the nuclear polarization as, for example, described by Timsit et al. in "Nuclear Polarization Measurements of Oriented .sup.3 He Gas by `Frequency Jump` Spectrometry," Review of Scientific Instruments, Vol. 47, No. 8, August 1976. However, in relating these frequency jumps to the nuclear polarization, standard pulsed NMR techniques are employed involving an amplitude measurement originating from signals induced in a pickup coil. Thus, the calibration of the frequency jump ultimately rests upon an amplitude measurement which is itself susceptible to the same disadvantages present in the bridge and marginal oscillator techniques discussed heretofore.